This invention relates generally to a method and means for controlling the power factor, the phase angle between current and voltage, of alternating current (AC) transmission lines supplying power to a time varying load and more particularly to a static var (volt-amperes reactive) generator which functions as a variable reactance utilized for load compensation and power factor correction on such transmission lines.
While rotating synchronous condensers and banks of mechanically switched fixed capacitors or inductors have been used in the past for var compensation and power factor correction, recent advances in high power thyristor technology have resulted in the development of controllable static var sources, commonly referred to as var generators. One known general class of var generators, known as current source var generators, comprises a six pulse AC to DC converter with the AC terminals connected to the AC transmission lines and the DC terminals shorted through a DC inductor. A particular type of that class utilizes for its six pulse AC to DC converter the inverter section of a controlled current inverter (CCI).
As disclosed in a publication entitled, "Reactive Power Generation and Control by Thyristor Circuits" by Laszlo Gyugyi, which appeared in the IEEE Transactions On Industry Applications, Vol. 1A-15, No. 5, September/October, 1979, pp. 521-532, a current source inverter utilized as a var generator may be either naturally commutated or force commutated. The naturally commutated inverter can only provide lagging vars, while the force commutated inverter can provide both lagging and leading vars. A naturally commutated inverter is comprised of a simple bridge of six thyristors, while a force commutated inverter includes a bridge of six thyristors, series connected diodes and cross coupling capacitors, both circuits being well known to those skilled in the art. A naturally commutated inverter can only operate when the thyristors are fired at such delay angles where the DC current is naturally transferred from one pair of thyristors to the next and thus operates as a controllable rectifier where the delay angle of thyristor firing is restricted to the range of 0.degree. to 180.degree.. By gating the thyristors near 90.degree. lagging; that is, where the AC current lags the voltage by 90.degree.; a voltage of nearly zero volts DC will be generated and the current in the inductor can be controlled by slight changes (advances) in gating angle. Accordingly, the circuit appears as a continuously variable, balanced three phase inductor across the AC line, a source of controllable lagging vars.
In the force commutated inverter configuration, however, the thyristors act like gate turn-off devices capable of being operated over the total firing angle range of 0.degree.-360.degree. and can thus provide both leading and lagging line currents and, accordingly, can selectively act both as a variable balanced three phase capacitor or inductor.
While it is possible to control a force commutated thyristor bridge as a current source var generator (CSVG) in both leading and lagging quadrants, it inherently exhibits a control characteristic which is highly non-linear, includes very large and variable transport delay, and has different control characteristics in leading operation as opposed to lagging operation.